(1) Field of the Invention
This invention relates to a sulfur(S-containing polyurethane base lens resin having a high refractive index and good machinability such as high degrees of easiness in its cutting and polishing (hereinafter called "cutting and polishing easiness" for the sake of brevity).
(2) Description of the Prior Art
In Japan, more and more plastic lenses have recently been employed as eyeglass lenses and camera lenses and in optical devices, since they are lighter and less fragile compared with inorganic glass lenses and permit coloration. As a representative resin employed widely for the above purposes these days, may be mentioned a radical polymerization product of diethylene glycol bisallyl carbonate. The radical polymerization product will hereinafter be abbreviated as "DAC resin". Although the DAC resin has various advantages such as excellent impact resistance, reduced temperature dependency of lenses' refractive powers, light weight, superb colorability, good machinability such as good cutting and polishing easiness, etc., it cannot be considered to have sufficient properties as a resin for lenses in view of the recent trend toward fashion-oriented lenses in the field of eyeglass lenses.
As the most serious drawback of the DAC resin, its refractive index is smaller compared with those of inorganic lenses (refractive index of a typical inorganic lens, N.sub.D.sup.20.degree. C. : 1.52; refractive index of a DAC resin lens N.sub.D.sup.20.degree. C. : 1.50). When the DAC resin is molded into a lens, the lens has a greater thickness. In the case of high refractive-power eyeglass lenses for the near-sighted in particular, the lenses have great peripheral thicknesses. Use of the DAC resin cannot achieve weight reduction and moreover results in eyeglass lens of poor visual attraction. Accordingly, lenses which make use of the DAC resin as a raw material are pronely shunned due to the recent tendency to place importance on fashionability.
For the reasons mentioned above, there is an outstanding demand for a lens resin having a higher refractive index than the DAC resin, in other words, capable of providing thinner lens thicknesses than the DAC resin.
Certain urethane base resins have been known as typical lens resins capable of achieving high refractive indexes. For example, there has been known a urethane resin obtained by a reaction of an isocyanate compound with a hydroxyl-containing compound such as diethylene glycol (Japanese Patent Laid-Open No. 136601/1982 or 136602/1982) or with a halogen- and hydroxyl-containing compound such as tetrabromobisphenol A (Japanese Patent Laid-Open No. 164615/1983).
These urethane-base resins are however unable to obtain refractive indexes beyond a certain level. In order to obtain a resin having a refractive index, N.sub.D.sup.20.degree. C., in the neighborhood of about 1.60 or higher, it is indispensable to use an aromatic isocyanate and/or a monomer containing many halogen atoms as substituents therein. However, use of such monomers leads to a drawback in external appearance that the resultant resin is colored and another drawback in chemical and physical properties that the resultant resin has poor weatherability and poor cutting and polishing easiness.